Blender 2.8 Crashed and cannot reopen the project anymore
While I can't tell you what happened exactly, I did find a way to recover your sculpt.

What seems to have happened is that somehow the object got removed, but the mesh data still stayed (I believe the technical term here would be Orphan Data). How that happened I'm not sure, but I tried appending the mesh again, and switched to the Data API view in the outliner and noticed it said 0 objects but 1 mesh.

The way to recover your object is to add another mesh object, go to the Mesh Data tab of the properties panel, and right at the top where it says the name of the object, on the left you can select different mesh data. Select the name of the object you're trying to recover and whatever mesh data the object you added had will now be replaced by the sculpted object.